1. Field of the Invention
This invention relates to polyimides, more particularly to colourless and low colour polyimides, and a method for their preparation.
2. Description of the Background
Colourless polymers are used in a wide range of applications. Many of these materials lie between commodity and speciality polymers and find use in application areas where cost remains a prime consideration. These types of polymer include acrylics and polycarbonates. They find use in applications as diverse as packaging and headlamp lenses. Their temperature capability is normally limited to no more than 100.degree. to 120.degree. C.
Another polymer category where low colour is required in some applications is high temperature polymers. An increasingly important field of application of colourless high temperature polymers is in optoelectronics. High temperature capability is needed because of the high power rating of modern high density microelectronic circuitry. Other opportunities exist in the coatings field. Few colourless high temperature polymers exist because the chemical features which lead to high thermal stability often result in coloured products. A number of approaches have been adopted to make colourless polymers such as polyimides. In general, these approaches lead to a significant increase in the cost of the polymers.
Much of the prior art concerning colourless high temperature polymers is concerned with the use of fluorine-containing substituents. Early work of Polaroid focused on the incorporation of trifluoromethyl (TFM) groups in the meta positions of biphenyl units in the main chain of polyamides. The bulky TFM groups prevent coplanarity of the adjacent phenyl rings in the biphenyl units, thus hindering the interactions which lead to colour development. The synthesis of the TFM-containing monomer is complex, leading to relatively high cost of the polymer.
Colourless or low colour polyimides have been studied in depth by NASA (see, for example, T. L. St. Clair in "Polyimides", Eds. D. Wilson, H. D. Stanzenberger and P. M. Hergenrother, Blackie, 1990). The main approaches to reducing colour have been to introduce groups which either disrupt extended conjugation along the polymer backbone, or which eliminate the electronic interactions such as charge transfer complexation which lead to colour. A. K. St. Clair and T. L. St. Clair have described in U.S. Pat. No. 4,603,061 the use of diamine and dianhydride monomers containing bulky electron-withdrawing groups and separator groups to reduce the colour in the resultant polyimides. The majority of colourless or low colour systems reported contained the hexafluoroisopropylidene group. The same inventors described in U.S. Pat. No. 4,595,548 the preparation of low colour polyimides containing phenoxy or thiophenyl linking groups in the polymer backbone. A. K. St. Clair and W. S. Slemp (23rd. Int. SAMPE Tech. Conf., 21-24 Oct., 1991, p. 817) also described the use of phenoxy-containing monomers to provide colourless or low colour polyimides with good optical transparency for use in space applications. The polymers had Tg values in the range 175.degree. to 275.degree. C. Another phenoxy-containing polyimide which is described as colourless is disclosed in JP 62185715 (1987) to Mitsui-Toatsu. This is produced by reaction of oxydiphthalic anhydride (ODPA) with oxydianiline (ODA).
Nitto Electric Industrial Co. Ltd. of Japan have also developed a colourless, transparent aromatic polyimide based on biphenyl dianhydride and sulphone-containing diamines (T. Inoue, Polymer Yearbook 5, Harwood Academic Publishers, London, 1989, p. 305). Nitto Electric have also described materials based on similar colourless polyimides for use in solar cell protection (JP 63027579, 1988). The use of colourless polyimides for use in LCD applications is described in JP 63232205 (1988). Nitto have also disclosed colourless, transparent polyimide mouldings for use as coverings for semiconductor devices (JP 2003991, 1990). These materials appear to be based on polymers containing hexafluoroisopropylidene groups.
Other colourless, fluorine-containing aromatic polyimides have been described in T. Omote, T. Yamaoka and K. Koseki, J. Appl. Polym. Sci., 1989, 38, 389. These are designed for use as photoreactive precursors in microelectronics applications. Hitachi have described (D. Makino, ACS PMSE Preprints, 1992, 66, 233) the use of colourless fluorinated polyimides in optical waveguide applications. Recently, other polyimides containing the perfluoroisopropylidene group have been described by NASA as having low colour (High-Tech Materials Alert, April 1993, p. 2). These polyimides are soluble in organic solvents and form tough, flexible films and coatings.
It is clear from the above that most of the work on colourless or low colour polyimides has been undertaken on predominantly aromatic systems. Although polyimides containing aliphatic groups in the main chain have been reported, to the best of our knowledge none of these have been concerned with the colour of the polymer. B. A. Zhubanov et al (in Polyimides and other High-Temperature Polymers, Eds. M. J. M. Abadie and B. Silion, Elsevier Science Publishers, Amsterdam, 1991, p. 455) described the synthesis of polyimides from alicyclic dianhydrides. No mention was made of the colour of these polymers. They seemed to be primarily of interest for their dielectric properties. B. Ramalingam et al (Third Int. conf. on Polyimides, 2-4 Nov. 1988, Ellenville, N.Y., p. 216) described the preparation of polyimides from diamines containing non-aromatic linkages. Tg values were generally well below 300.degree. C., the main aim being to improve processability. W. A. Feld and T-B Le (J. Polym. Sci., Polym. Chem. Ed., 1992, 30, 1099) have also described the synthesis of polyimides containing non-aromatic nitrogen linkages. These polymers were red or orange.